The invention relates to an X-ray apparatus which includes an X-ray source for producing X-rays, an X-ray detector for detecting the X-rays, and a filter which is arranged between the X-ray source and the X-ray detector and includes a plurality of tubular filter elements having a longitudinal direction and a circumference, wherein
each filter element has an internal volume for receiving a liquid filling which contains at least one electrically conductive and one X-ray absorbing liquid component, the X-ray absorptivity of said filter element being dependent on the quantity of X-ray absorbing liquid component present in the internal volume,
each filter element is provided with a first electrode for applying a first electric voltage to a wall of the filter element and a second electrode for applying a second electric voltage to the internal volume of the filter element,
the first electrode is electrically isolated from the internal volume of the filter element by means of an isolator layer in such a manner that an electric capacitance per unit of surface area of the filter element exists between the first electrode and the electrically conductive liquid component when a quantity of the electrically conductive liquid component is present in at least a part of the internal volume of the filter element,
the X-ray absorptivity of each filter element is adjustable by step-wise control of a surface level of the X-ray absorbing liquid component in the longitudinal direction of each filter element.
An X-ray apparatus with an X-ray filter of this kind is known from U.S. Pat. No. 5,666,396 (PHN 15.378). The known X-ray apparatus includes a filter with a plurality of filter elements, each having an individual absorptivity which is being dependent on the level of a liquid filling present in the filter element. The X-ray apparatus is used inter alia for medical diagnostic imaging where a patient to be examined is arranged between the X-ray source and the X-ray detector in order to image internal structures. Because of the fact that there are structures of different electron density present within the patient, areas of different density are observed in a resultant X-ray image. The difference in density between the extreme values of the density in one X-ray image is defined as the dynamic range. The filter serves to limit a dynamic range per X-ray image.
In order to limit the dynamic range of the object to be examined, the known X-ray apparatus includes a filter with filter elements provided with a bundle of tubes for receiving a liquid filling which is X-ray absorbing as well as electrically conductive; each tube is connected to a common supply duct. Each filter element is provided with a first electrode which is arranged in a wall of the filter element in order to apply an electric voltage to the wall of the filter element. A second electrode is in contact with the liquid filling. The electric voltage applied to the first electrode of the filter element influences the adhesion between the liquid filling and an inner wall of the filter element; this adhesion determines whether the relevant filter element is filled with the liquid filling. The relative quantity of the liquid filling in individual filter elements is controlled on the basis of the electric voltages applied to the individual filter elements. For example, for a first value of the electric voltage the adhesion to the inner wall for the liquid filling is increased and the relevant filter element is filled with the liquid filling from the supply duct. For a second value of the electric voltage the adhesion is reduced and the liquid filling is drained from the filter element to the supply duct. The filter elements are adjusted to a high X-ray absorptivity by filling them with the liquid filling; they are adjusted to a low X-ray absorptivity by keeping filter elements empty.
It is a drawback of the known device that the filling of each filter element is controlled by application of a sequence of electric voltage pulses to the first electrode of the filter element so that the filter element is electrically charged. The level of the electric charge determines the degree of filling of the filter element. It has been found that in the course of time the filling level of the filter element becomes poorly reproducible. In many practical cases it is desirable to have a reproducible filling with an a priori known degree of discretization so as to realize a reliable range of gray scale values.
It is an object of the invention to provide an X-ray apparatus which includes a filter provided with filter elements whose X-ray absorptivity can be controlled in steps.
An X-ray apparatus according to the invention is characterized in that the electric capacitance per unit of surface area of the wall of the filter element varies substantially in the longitudinal direction of the filter element.
The invention utilizes the known effect that a contact angle between an electrically conductive liquid and an electrode which is isolated therefrom is changed by creating a potential difference between the electrically conductive liquid and the electrode. This phenomenon is known as electrowetting. When electrowetting is applied to a tubular filter element which has an electrode provided in its wall and is filled with an electrically conductive liquid filling, the level of said liquid filling in the filter element can be influenced due to the fact that the electrowetting force is oriented in the longitudinal direction of the filter element so that the degree of filling of the filter element can be increased or decreased at option. In order to realize the potential difference between the liquid filling and the first electrode provided in the wall of the filter element, they are electrically isolated from one another by means of an isolator layer deposited on the inner wall of the filter element. In order to achieve a low wetting hysteresis, the isolator layer may also be covered by an inert cover layer so that the liquid filling directly contacts the cover layer. A capacitance per unit of surface area of the filter element which is due to the geometry of the filter element can then be defined. It is known that in electrowetting a balance exists between the capillary force, the force of gravity and the electric capillary force or electrowetting force. The relationship between the level of the electrically conductive liquid filling in the filter element and the relevant physical quantities can be derived from the equations for the energy balance in the filter element. There can be defined two dominant variables whose value determines the electrowetting force in the filter element. The first variable is the capacitance per unit of surface area which is averaged over the length of the contact edge between the meniscus of the electrically conductive liquid filling and the inert cover layer. The second variable is the potential difference between the electrically conductive liquid filling and the first electrode. The invention is based on the idea to realize the filter element filling in steps by step-wise varying the capacitance, and hence the minimum potential difference necessary for electrowetting to occur, in the longitudinal direction of the filter element.
The procedure for the step-wise filling of the filter element is as follows. The operation of the filter element will be described first of all for the case where the liquid filling contains two liquid components that can fully dissolve in one another, thus forming one electrically conductive and X-ray absorbing liquid. It is also assumed that the filter element is empty, that the capacitance variation profile in the longitudinal direction of the filter element is known a priori, and that no potential difference exists yet between the liquid filling and the first electrode. Finally, a distinction is made between a xe2x80x9cfillxe2x80x9d voltage for completely filling the filter element, a xe2x80x9choldxe2x80x9d voltage for keeping the liquid filling in position, and a xe2x80x9cdrainxe2x80x9d voltage for draining the filter element. The duration of the voltage pulse of the xe2x80x9cdrainxe2x80x9d voltage or the xe2x80x9cfillxe2x80x9d voltage determines the volume of the filling. A relevant control chart is as follows: during step one a voltage is applied to the first electrode in such a manner that all filter elements are filled (the xe2x80x9cfillxe2x80x9d voltage). Subsequently, the voltage for all filter elements is lowered to the xe2x80x9choldxe2x80x9d voltage. Finally, per individual filter element the pulses of the xe2x80x9cdrainxe2x80x9d voltage are applied with a pulse duration such that the liquid filling is lowered to the required level.
It is alternatively possible to form the liquid filling from more, notably two, liquid components which are not miscible. In that situation, for example the properties of the liquid components can be individually optimized so that, for example, one liquid component has optimum electrical conductivity properties and hardly absorbs X-rays whereas the second liquid component has optimum X-ray absorbing properties and is electrically insulating. This situation can also be used to make one of the liquid components electrically conductive as well as X-ray absorbing and to choose the second liquid component to be such that it prevents degradation of the inert cover layer. The respective liquid columns may then be contiguous so that a common interface is formed in the transverse direction. However, it is also feasible for the two liquid components to remain separated by a gas layer. Furthermore, it must be possible to supply the liquid components from a respective supply duct. In that case the filter element is always filled with the liquid filling, the degree of the X-ray absorption being determined by the level of the X-ray absorbing liquid component in the filter element. The operation of the filter element is then similar to that according to the described control chart. According to this method the level of the X-ray absorbing liquid component is determined passively by the level of the electrically conductive liquid component in the filter element and the maximum X-ray absorption is reached when the filter element is completely filled with the X-ray absorbing liquid component.
One method of varying the mean capacitance per unit of surface area consists in locally changing the surface area of the electrode relative to the surface area of the tubular filter element, for example by means of electrode constrictions. To this end, a first embodiment of the X-ray apparatus according to the invention is characterized in that the first electrode includes a number of electrically interconnected first and second electrode segments, each of which extends at least over a part of the circumference of the tubular filter element, the first and the second electrode segments being arranged so as to succeed one another in the longitudinal direction of the filter element, and that the first electrode segment extends over a larger part of the circumference of the filter element in comparison with the second electrode segment.
Another method of varying the capacitance per unit of surface area consists in the use of a number of different isolator materials. A further embodiment is characterized in that the isolator layer includes a number of first and second isolator segments, the first and second isolator segments succeeding one another in the longitudinal direction of the filter element, the first isolator segment having a dielectric constant which is higher than that of the second isolator segment.
It is alternatively possible to vary the capacitance per unit of surface area by varying a distance between the liquid filling and the first electrode. This is the case in a third embodiment which is characterized in that the isolator layer includes a number of first and second isolator layer segments, the first and second isolator layer segments succeeding one another in the longitudinal direction of the filter element and the first isolator layer segment having a layer thickness which is larger than that of the second isolator layer segment.